For context, child is 3.8 (4 this summer), the parent is my friend who was out of sight at the far end of the garden (it's a big garden) making a phone call. Cat is 15 with arthritis and was snoozing in the sun.
I was inside washing up and looking out if the closed kitchen window when I saw child looking down at floor and moving strangely. I stood on tiptoes and moved to the side to get a view of child's feet and realised they were trying to step on the cat's swishing tail. I yelled "[CHILD] STOP IT, YOU DO NOT STEP ON THE CAT'S TAIL" extremely loudly through the window just as the child's foot came down on the end of cat's tail. Child jumped backwards then looked round to see who had shouted. I went and opened back door, crouched down and said "[Child] I need to talk to you" at normal volume and in a normal tone of voice but child ran off toward friend who had heard me shout and come to see what the problem was.
Friend has now got the hump and gone home because a) they don't shout and b) child claims not to have been stepping on tail. I don't usually shout either especially not at other people's DC, but child was outside, no parent in sight, windows and doors shut, and I was worried for my poor old cat and wanted to intervene fast enough to prevent a tail injury!
I am absolutely 100% certain that's what child was doing, I would never have shouted had I not been certain.
So how would anyone else have reacted?